Moment of Inertia, and Stress Area. 431 To find the Moment of Inertia of any Beam Section. -Proceed as in the last construction and find Z. nd I = Zjy.* So for rectangular sections bJP h ^bh* 62 12 Then Z = - y . \ — X --- Stress areas for circle, hollow circle, triangle, and hollow •ectangle we shewn in Fig. 385, being measured as in Fig. 372 mean width x - Y The centre of gravity of each area is obtained y cutting out in stiff paper and hanging up in two different ,|tf_ - .....O —— —- —y >ositions to mark two plumb lines, which will cross at G. For he triangular beam the neutral axis must be drawn at ^ the leight, a line of limiting stress drawn across the apex, and another >elow, at an equal distance from its axis. Projecting and con- urging, we shall obtain the areas shewn, which must be equal. The results are as follows : Moment of resistance of circle =f'og82 d* of hollow circle =7-0982 : D of triangle =7-0417 of hollow rectangle =7'1666: H * See also note, page 430; see also page 845.